Describing color is a subjective phenomenon, involving the perspective of the observer and the difficulties in verbally expressing the characteristics of the color, such as reddish-green or bluish-green or the like. To avoid the problems inherent in individual color observations, a number of objective color scales have been developed, wherein colors are expressed in numerical values according to various coordinate systems. One of the earliest color systems was developed in 1905 by Albert Munsell. This system, which is still used today, is called the Munsell System of Color Notation. The Munsell system assigns numerical values to three properties of color: hue, value, and chroma. Hue is generally thought of as the color (red, green or blue or combination) that one would observe. Value is the measure of the lightness or darkness of a color, with color values running from black to white. Chroma is a measure of the intensity or saturation of a color. Munsell developed a three dimensional physical model representing the various colors, with the colors or hues being arranged in continuous form in a circle, value extending along the axis of the circle and chroma extending radially outwardly from the axis of the circle.
Today there are mathematical scales representative of hue, value, and chroma properties of colors. The most important of these is a standardized system developed by the International Commission on Illumination ("CIE"). This organization has developed two principal color scales for numerically expressing color values. These scales are the CIEIAB and CIELUV scales. These scales are mathematically related and are in turn related to a so-called "tristimulus" scale, wherein colors are expressed in terms of X, Y, and Z coordinates. The CIELAB and CIELUV scales are used in preference to the tristimulus XYZ coordinates, because the tristimulus coordinates are believed to correlate less accurately with the visual attributes of a color. Standardized color scales such as CIELAB are widely used in industrial applications and are the standards in the industry.
Instruments generally used for color analysis include the spectrophotometer, colorimeter, and densitometer. These instruments vary in the manner in which they produce output, but all basically produce a numerical analysis of an average color on a viewing surface in terms of three principal components. The data produced by these instruments can be expressed in terms of tristimulus coordinates, CIELAB coordinates or coordinates of other related color scales.
An improved method for analyzing and comparing colors involves the use of color video cameras and computer digitizing equipment. With this equipment, colors are viewed by a video camera and digitized into their respective color values, typically numeric values representative of red, green and blue values ("RGB"). These numeric values can be manipulated for color analysis and comparison in the same manner as the more standard CIELAB or tristimulus values. The use of video equipment provides important additional data however, since the video equipment is capable of producing component values over a given area on a pixel by pixel basis, and the varying colors represented by the different pixel locations can be analyzed and compared on a statistical basis to produce data representative of the color variation patterns over a viewing surface. Applicant's U.S. Pat. No. 4,812,904 is illustrative of such a system.
When RGB values are developed by a video camera, the RGB scale for each video camera tends to have a unique bandwidth configuration, which can vary in significant respects from the RGB scale of a different camera. Actually, the reference to RGB signals in such applications really means that the instrument breaks down energy readings in the visible spectrum to three distinct bands. As used herein, the term "RGB" or "video RGB" refers to the general class of RGB scales developed by different instruments. RGB scales can be developed by instruments such as colorimeters in addition to video cameras, but the video camera has broader capabilities than the colorimeter.
RGB values developed by video cameras are perfectly acceptable from a functional standpoint as a means for numerically describing and comparing colors. In such a case, each individual camera is calibrated by reference to standard color samples, so the unique variations among color scales has no effect. The variations in RGB scales, however, precludes the use of a single formula or set of data for expressing the video RGB values in any standard scale or translating RGB values to a different standard color coordinate system, such as tristimulus XYZ values or CIELAB or CIELUV. RGB values thus encounter some resistance from persons who feel more comfortable using industry standard tristimulus or CIELAB values.
There is a so-called "standard" RGB scale, but such a scale cannot be produced with conventional video equipment. The standard RGB scale can only be developed by special equipment with very precise sensors of a particular nature found in filter colorimeters. There are known formulae for converting standard RGB values to standard tristimulus values, but there is no known method for converting RGB scales in general to tristimulus coordinates.
The purpose of the present invention is to develop a method for converting or correlating numerical RGB values developed by different instruments into standard tristimulus values. Tristimulus values then can be converted by known mathematical formulae to any of the desired standardized color scales in use in industry.
The present invention provides a system in which a video camera may be used to obtain standard color values from a desired object. More particularly, the present invention provides a system by which a video camera may be used to generate RGB values from an imaged object, and the RGB values so generated may then be converted to standard values. A salient attribute of the present system is that it allows different video cameras which produce non-standard RGB values to output uniform values.
In a still more particular sense, the present invention uses iterative regression analysis to determine initial functions which convert RGB values generated by a video camera from initial colors to standard XYZ values. Regression analysis is then used to determine additional functions which convert RGB values generated by the video camera viewing additional colors different than the initial colors to standard XYZ values. The functions generated for the video camera are then used to convert RGB values generated by the video camera in imaging a colored object to standard XYZ values. Accordingly, the system of the present invention enables the use of an RGB video camera to generate signals representing the standardized color codes for the object imaged by the camera.
These and other features and attributes of the invention will become more apparent after contemplation of the ensuing more detailed description.